Reactive Polymers Fundamentals and Applications. A Concise Guide to Industrial Polymers

531 Pages·2013·7.247 MB·English

by  Johannes Karl Fink (Auth.)

1 Unsaturated Polyester Resins
Unsaturatedpolyesterresinsconsistoftwopolymers, 1.2.1 Monomersforan
i.e., a short-chain polyester containing polymeriz- UnsaturatedPolyester
able double bonds and a vinyl monomer. The curing
Monomersusedforunsaturatedpolyestersareshown
reaction consists of a copolymerization of the vinyl
in Table 1.1 and in Figures 1.1 and 1.2. Unsaturated
monomer with the double bonds of the polyester. In
diolsareonlyrarelyused.
the course of curing, a three-dimensional network
is formed. Unsaturated polyester resins belong to
1.2.1.1 Alcohol Components
the group of so-called thermosets. There are several
monographs and reviews on unsaturated polyesters The most common alcohol components are 1,2-
andunsaturatedpolyesterresins[1–7]. propyleneglycolandethyleneglycol(EG).Ethercon-
We will differentiate between unsaturated taining alcohols exhibit better air-drying properties
polyesters and unsaturated polyester resins. Unsat- andareusedintopcoats.Polyestersbasedonunsatu-
urated polyesters are the polyesters as they emerge rated diols can be prepared by the transesterification
from the condensation vessel. They are rarely sold of diethyl adipate with unsaturated diols, e.g., cis-
assuch,becausetheyarebrittleatroomtemperature 2-butene-1,4-diol and 2-butyne-1,4-diol. The trans-
anddifficulttohandle.Instead,wheneverapolyester esterification method is a suitable procedure for the
is freshly synthesized in a plant, it is mixed with the preparation of unsaturated polyesters in comparison
vinyl monomer in the molten state. Thus materials tothedirectpolycondensation[11].cis-2-Butene-1,4-
that are viscous at room temperature, with a styrene diol,themostavailablealiphaticunsaturateddiol,has
content of ca. 60%, are sold. Such a mixture of an beenusedtoproducesomevaluablepolymerssuchas
unsaturated polyester with the vinyl polymer is graftableunsaturatedsegmentedpoly(urethane)sand
referredtohereasanunsaturatedpolyesterresin. crosslinkablepolyestersformedicalpurposes.
Chemically modified soybean oil is an inexpen-
1.1 History sive alternative candidate for unsaturated polyester
(UP)compositions.Inaddition,whenreinforcedwith
Itwasrealizedlongagothatsomenaturaloilsaswell natural fibers, these composites could yield compa-
asalkyleresinscanbedriedbycertainadditivesand rable and adequate properties to common products.
used as coatings. This drying results from a poly- Acrylated epoxidized soybean oil or maleated acry-
merization of the unsaturated moieties in the ester latedepoxidizedsoybeanoilcanbeusedforthesyn-
molecules. Next it was discovered that the addition thesis of a prepolymer that contains vinyl groups.
ofstyrenewouldacceleratethedrying.Theinvention These groups are then copolymerized with styrene,
ofunsaturatedpolyesterresinsisascribedtoCarleton asotherwiseusual[15].
Ellis (1876–1941). The first patents with regard to
polyester resins emerged in the 1930s [8–10]. Com- 1.2.1.2 Acid and Anhydride
mercialproductionstartedin1941alreadyreinforced Components
with glass fibers for radar domes, also referred to as
Ageneral-purposeindustrialunsaturatedpolyesteris
radomes.
madefrom1,2-propyleneglycol,phthalicanhydride,
and maleic anhydride. The most commonly used
1.2 Monomers vinylmonomerisstyrene.Maleicanhydridewithout
phthalicanhydridewouldyieldapolyesterwithahigh
According to the composition of an unsaturated
density of double bonds along the polyester chain.
polyesterresin,themonomerscanbegroupedintwo
This would result in a high crosslinking density of
main classes, i.e., components for the polyester and
thecuredproduct,thusinabrittleproduct.Therefore,
componentsforthevinylmonomer.
theunsaturatedacidcomponentisalwaysdilutedwith
Fink:ReactivePolymersFundamentalsandApplications.http://dx.doi.org/10.1016/B978-1-4557-3149-7.00001-2
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2 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS
Table1.1 MonomersforUnsaturatedPolyesters
SaturatedAlcohols Remarks
1,2-Propyleneglycol Mostcommonglycol
Ethyleneglycol Lesscompatiblewithstyrenethanpropyleneglycol
Diethyleneglycol Gooddryingproperties
Neopentylglycol Goodhydrolysisresistance
Glycerol Trifunctional alcohol, for branched polyesters. Danger of crosslinking
duringcondensation
Flameretardant
TetrabromobisphenolA(TBBPA)
Trimethylolpropane Trifunctionalalcohol,cheaperthanglycerol
Trimethylolpropanemonoallylether Weatherresistantforcoatings[12,13]
Undecanol Usedaschainstopper
SaturatedAcidsandAnhydrides Remarks
Phthalicanhydride Mostcommonanhydride
Isophthalicacid Goodhydrolysisresistance
Terephthalicacid Superiorhydrolysisresistance
HETacid Flameretardantsystems.Infact,evenwhenaddressedasHETacid,
theHETanhydrideisused
Tetrabromophthalicanhydride Flameretardantsystems
Adipicacid Softresins
Sebacicacid Softresins
o-Carboxyphthalanilicacid [14]
UnsaturatedAcidsandAnhydrides Remarks
Maleicanhydride Mostcommon
Fumaricacid Copolymerizesbetterwithstyrenethanmaleicanhydride
Itaconicacid
CH willnotpolymerize.Fumaricacidcopolymerizeswell
3
HO C H C H OH HO CH CH OH with styrene, but fumaric acid is more costly than
2 2 2
maleicanhydride.Therefore,maleicanhydrideisthe
Ethylene glycol Propylene glycol preferredunsaturatedacidcomponent.Anotheraspect
isthatduringthecondensationoffumaricacid,2mol
ofwatermustberemovedfromthereactionmixtures,
CH
3 whereas in the case of maleic anhydride only 1mol
HO CH C CH OH
2 2 of water must be removed. Anhydridesare preferred
CH3 over the corresponding acids because of the higher
reactivity.
Neopentyl glycol
Isophthalic acid and terephthalic acid cannot form
an anhydride. These compounds do not condense as
CH2OH fast as phthalic anhydride. On the other hand, the
H3C CH2 C CH2OH CH2 CH CH2 polyestersfromisophthalicacidandterephthalicacid
CH2OH OH OH OH aremorestablethanthosemadefromphthalicanhy-
dride. That is why these polyesters with neopentyl
Trimethylol propane Glycerol glycolareusedinaggressiveenvironmentsandasgel
coats and top coats. A gel coat is the first layer of a
Figure 1.1 Diols and triols used for unsaturated
polyesterresins. multilayer material; the top coat is the layer on the
oppositeside.Forinstance,ifapolyesterboatisbuilt,
an acid with non-polymerizable double bonds. Note the gel coat is first painted into the model. Then a
that aromatic double bonds also will not polymerize seriesofglass-fiber-reinforcedlaminatesareapplied,
withvinylcomponents.ThedoublebondinHETacid andfinallythetopcoatispainted.
1:UNSATURATED POLYESTER RESINS 3
O COOH step, the intermediate reacts with maleic anhydride
and with 1,2-propylene glycol. The resulting unsat-
O urated polyester resin has a fumarate content greater
thanabout85%[17].Thehighfumaratecontenthelps
O COOH the resins to cure quickly and thoroughly with vinyl
monomers, giving the resulting thermosets excellent
Phthalic anhydride Isophthalic acid waterresistance.
Cl Cl 1.2.1.3 Amine Modifiers
Cl
COOH Cl Theadductsofethyleneoxideorpropyleneoxidewith
O N,N(cid:3)-diphenylethane-1,2-diamine or N,N-dimethyl-
p-phenylene diamine and ethylene oxide (EO) with
Cl Cl O N,N(cid:3)-diphenylhexane-1,6-diamine can be used as
modifiers.Whenusedinamountsupto2%,theamines
COOH O substantiallyreducethegelationtimeofthesemodi-
fied unsaturated polyesters. However, as the reactiv-
Terephthalic acid HET-anhydride
ity of the resins increases, their stability decreases
[18,19].
O
H
1.2.1.4 Dicyclopentadiene
O
COOH Dicyclopentadieneisusedinawidevarietyofappli-
H CH CH cations,includingelastomers,flameretardants,pesti-
O HOOC
cides, and resins for adhesives, coatings, and rubber
tackifiers. Approximately 30% of the production is
Maleic anhydride Fumaric acid
usedforunsaturatedpolyesterresinsbecauseoftheir
Figure 1.2 Acids and anhydrides used for unsatu-
valuableproperties[20].
ratedpolyesterresins.
Dicyclopentadienepolyesterresinsaresynthesized
fromdicyclopentadiene,maleicanhydride,andagly-
col.Thereactionisperformedinthepresenceofwater
Isomerization. During the synthesis of the poly- togeneratemaleicacidfromthemaleicanhydrideto
ester, maleic anhydride partly isomerizes to fumaric form dicyclopentadiene maleate. The ene reaction is
acid.Theisomerizationfollowssecond-orderkinetics showninFigure1.3.
becauseofthecatalysisbymaleicacid.Theactivation Themaleateisesterifiedwiththeglycoltoformthe
energyoftheisomerizationisca.63.2kJmol−1 [16]. unsaturated polyester resin [21,22]. The ene adduct
2-Methyl-1,3-propanediol offers significant pro- servestoformendcappedpolyesters.Athighertem-
cess advantages to resin producers because it is an peraturesdicyclopentadieneundergoesaretroDiels-
easilyhandledliquid,ithasahighboilingpoint,andit Alder reaction and can add to the unsaturations of
hastwoprimaryhydroxylgroupsforrapidcondensa- fumaric acid and maleic acid (as pointed out in
tions. Polyester resins produced from 2-methyl-1,3- Figure 1.4), to form nadic acid units. When the
propanediol using conventional condensation poly- dicyclopentadiene-modified unsaturated polyester is
merization, however, have relatively low fumarate used for a molding material, the polyester is usually
contents (60–70%), and simply increasing the mixedwitharadicallypolymerizablemonomeranda
reactiontemperaturetopromoteisomerizationcauses polymerization initiator. This allows the viscosity or
colorproblems. curingtimeofthemoldingmaterialtobesuitablefor
The two-step process helps increase the degree of themoldingoperation.
isomerization for such systems. First, the aromatic Dicyclopentadiene-modified unsaturated poly-
dicarboxylic acid is allowed to react with 2-methyl- esters yield molded articles with excellent perfor-
◦
1,3-propanediol at a temperature of up to 225 C mance.Thefunctionofdicyclopentadieneistoimpart
to produce an ester diol intermediate. In the second air-dryingcharacteristics,low-profileproperties,high
4 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS
O O Table1.2 VinylMonomersforUnsaturated
OH OH PolyesterResins
+ H
Monomer Remarks
OH
O
Styrene Mostcommon,butcarcinogenic
O O
p-Vinyltoluene Notreallyasubstituteforstyrene
Figure 1.3 Ene reaction between maleic acid and α-Methylstyrene Slowsthecuring
dicyclopentadiene. Methylacrylate
Methylmethacrylate Goodopticalproperties
Diallylphthalate
Triallylcyanurate
CH CH
2
CH CH
2
O O
O O
CH
3
+
O O
Styrene p- Vinyltoluene
O O
Figure 1.4 Retro Diels-Alder reaction ofdicyclopen-
tadieneandDiels-Alderreactionbetweenmaleicacid CH3
unitsandcyclopentadiene. C CH
2
CH
2 O
heat distortion, excellent weathering performance, CH2 CH2 C
O CH
and increased filler dispersibility in the resulting 3
polymer[23].
α-Methylstyrene Methyl methacrylate
1.2.2 VinylMonomers
Thevinylmonomerservesassolventforthepolyester O
and reduces its viscosity. Further, it is the agent C O CH2 CH CH2
of copolymerization in the course of curing. Vinyl
monomersforunsaturatedpolyesterresinsareshown
inTable1.2andinFigure1.5. C O CH2 CH CH2
O
1.2.2.1 Styrenes
Diallyl phthalate
Styrene is the most widely used vinyl monomer Figure1.5 Vinylmonomersforunsaturatedpolyester
for unsaturated polyesters. However, styrene has a resins.
carcinogenic potential: therefore, replacing styrene
bysomeothervinylmonomerhasbeendiscussedfor
fibers, which is useful in preventing delami-
years.
nation.
With larger amounts of styrene the rigidity of the
materialcanbeincreased.α-Methylstyreneformsless
1.2.2.2 Acrylates and Methacrylates
reactive radicals, and thus slows down the curing
reaction. Therefore, α-methylstyrene is suitable for Acrylates improve outdoor stability. Methyl meth-
decreasingthepeaktemperatureduringcuring. acrylate, in particular, enhances the optical proper-
Polar vinyl monomers, such as vinylpyridine, ties. The refractive index can be varied with mix-
improve the adhesion of the polyester to glass turesofstyreneandmethylmethacrylateclosetothat
1:UNSATURATED POLYESTER RESINS 5
CH CH O CH CH O CH CH cured products show good tensile properties and
2 2 2 2 2 2
O O weatherability.
CH CH
Another method used to make unsaturated
CH CH
2 2 polyesters water soluble is to introduce polar
hydrophilic groups such as carboxylic and sulfonic
Triethylene glycol divinyl ether
groups into the resin molecule, which ensures a
Figure1.6 Vinylethers. good dispersibility in water. An example of such
a compound is sodium 5-sulfonatoisophthalic acid.
Insteadofstyrene,glycerolmonoethersofallylalco-
of glass, so that fairly transparent materials can be hol and unsaturated fatty alcohols are used as vinyl
produced. monomer[27].
Unsaturated polyester resins diluted in water are
1.2.2.3 Vinyl Ethers used for particleboards and fiberboards. They are
modifiedwithacrylonitrileandusedasmixtureswith
Various vinyl and divinyl ethers have been used as
urea/formaldehyde (UF) resins. A mixture of a UP
substitutes for styrene. Divinyl ethers with unsatu-
resinandaUFresinallowstheproductionofboards
rated polyesters are used preferably in radiation cur-
which have considerably higher mechanical proper-
able compositions and coatings. However, special
tiesthanthosebondedexclusivelywithUFresins[28].
formulations containing no styrene but triethylene
glycoldivinylether(cf.Figure1.6)areavailablethat
canbeusedforgelcoats[24].
1.2.3.2 Low Emission Modifiers
Propenylethersaregenerallyeasiertopreparethan
Several methods have been proposed for reducing
theircorrespondingvinylethers.Thepropenylethers
volatileorganiccompoundemissions:
are simply prepared by isomerization of the corre-
sponding allyl ethers. Due to the steric effect of the
• addingskinformingmaterials,
methyl groups in the propenyl ether molecules, they
areexpectedtobemuchlessreactivethantheirvinyl • replacement of the volatile monomer with a less
etheranalogs[25].Examplesforpropenylethersare volatilemonomer,
ethoxylated hexanediol dipropenyl ether and 1,1,1- • reductionintheamountofthemonomerinthecom-
trimethylolpropanedipropenylether. positions,and
• increasing the vinyl monomers by attaching them
1.2.2.4 Other Vinyl Monomers
ontothepolyesterchain.
Triallylcyanurateenhancesthethermalstabilityofthe
finalproducts.Sincethecompoundistrifunctional,it The addition of paraffins to unsaturated polyester
enhancesthecrosslinkingdensity. resinshasbeenshowntoreduceemissionsbyforming
a thin film on the resin surface. While suitable for
1.2.3 Specialities some applications, this solution is unacceptable for
other applications due to a decreased interlaminate
1.2.3.1 Monomers for Waterborne adhesion[29].
Unsaturated Polyesters
Waterborneunsaturatedpolyestersareusedforwood Low Volatile Monomers. Styrene can be partly
coatings. They have UV-sensitive initiator systems. substituted by low volatile monomers. A num-
The basic constituents are selected from ethylene ber of alternatives have been sought to reduce
glycol, 1,2-propylene glycol, diethylene glycol, and emissions and as substitutes for styrene. Higher
tetrahydrophthalic anhydride, terephthalic acid, and molecular weight derivatives of styrene, such as
trimellitic anhydride [26]. The vinyl monomer is vinyl toluene and tert-butyl styrene, have been uti-
trimethylol propane diallyl ether. The UV-sensitive lized as reactive diluents and resulted in decreased
compound is 2-hydroxy-2-methylphenylpropane-1- emissions[29].
one. When diluted with water, the resins exhibit Bivalent metal salts of acrylic acid or methacrylic
a proper viscosity in the range of 2500cps. The acidhavelowvaporpressure.Examplesincludezinc
6 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS
diacrylate, zinc dimethacrylate, calcium diacrylate, fumaricacidwithepoxygroupsfromepoxideresins.
andcalciumdimethacrylate[30]. For example, n-hexanol reacts easily with maleic
The metal salt monomer is typically a solid, and anhydridetoformacidichexylmaleate.Thishalfester
therefore has much lower vapor pressure than, e.g., is then used for the addition reaction with the epoxy
styrene. The acrylate functionality copolymerizes resin[34].
readily with styrene. Due to the bivalent metal ions, Allyl alcohol in the unsaturated resins enhances
the acrylates act as crosslinkers of the ionomer type. their properties. The glass transition temperatures of
◦
Therefore, an additionalcrosslinkingoccurs in com- the epoxy fumarate resins exceed 100 C. The glass
parisontopurestyrene. transitiontemperaturesofepoxymaleatesarehigher
◦
than 70 C. The resins have good chemical resis-
Acrylate-modified Unsaturated Polyesters. tance[35].
Acrylate-modified unsaturated polyesters may be
used for low-viscosity resins and resins with low
emission of volatile monomers. In commercially
1.2.3.4 Isocyanates
availableunsaturatedpolyesterresinapplications,up
to 50% of styrene or other vinyl monomers are Isocyanates, such as toluene diisocyanate, can be
used. During curing some of the organic monomer added to a formulated resin, such as polyester plus
is usually lost to the atmosphere, which causes vinylmonomer.Thegelationtimesincreasewiththe
occupational safety hazards and an environmental concentrationoftoluenediisocyanate[36].
problem. During curing, toluene diisocyanate can ini-
Tailoring the polyester by synthesizing branched tially react with the UP moieties and produce
structures and incorporating additional vinyl poly(urethane) structures. Subsequently, intermolec-
unsaturations has been proposed. The diol alco- ularcrosslinkingoccurs.Toluenediisocyanatehasan
hols used for condensation may be partly replaced obviouseffectonthetoughnessandstrengthofaUP
by glycidyl compounds in order to obtain low- resin. A ratio of isocyanate to UP of 7.5% exhibited
molecular-weight methacrylate or acrylate-modified thebestmechanicalproperties[37].
or terminated polyesters [31]. Suitable glycidyl The viscosity increases strongly. Resins with only
compounds include glycidyl methacrylate and 3% of toluene diisocyanate are thixotropic [36]. An
glycidyl acrylate. Not more than 60mol% of the increaseintheviscosityishighlyundesirable.
alcohols can be replaced by glycidyl compounds A 1,6-hexane diisocyanate (HDI) trimer can
[24,32]. improvethemechanicalpropertiesofahydroxylter-
These polyesters have low viscosities because of minated UP type. The morphological analysis of the
the branched structures. In addition to the maleic surfaces of fracture reveals the presence of micro
or fumaric units, they bear additional unsatura- phase separated structures. These change from den-
tions resulting from the pending reactive acrylate dritic,todimpleandtocystiformstructureswhenthe
or methacrylate moieties. For this reason these contentoftheHDItrimerisincreased[38].
types need less vinyl monomer (styrene) to increase Toreducetheemissions,styrene-freeUPcomposi-
the crosslinking density of the cured product. The tionshavebeendeveloped[29].
increased unsaturation results in a higher reactivity, These are based on a dicyclopentadiene contain-
which in turn leads to an increase in heat distortion ingUP,aurethaneacrylateormethacrylate,amulti-
temperatureandbettercorrosionresistance,goodpig- functional methacrylate monomer, and a hydroxyl
mentability, and excellent mechanical and physical functionalizedmethacrylatemonomer.
properties [33]. Such resins are therefore suitable as Hydroxyl functionalized methacrylates may
basicresinsingelcoats. be used in the preparation of the urethane
methacrylatemonomercomponentorasamonomeric
reactivediluent.Thesecompoundsincludehydroxy-
1.2.3.3 Epoxide-Based Unsaturated
ethyl methacrylate, hydroxypropyl methacrylate,
Polyesters
hydroxyethyl acrylate, and hydroxypropyl acry-
Epoxide-based unsaturated polyesters are obtained late. The preparation of the base UP and the other
fromthereactionofhalfestersofmaleicanhydrideof monomershasbeenexemplifiedindetail[29].
1:UNSATURATED POLYESTER RESINS 7
1.2.3.5 o-Carboxy Phthalanilic Acid Alsopolymersfrommethylenediphenylisocyanate
havebeenusedinthecompositions.Theinfluenceof
A new acid monomer, o-carboxy phthalanilic
the modifications on thermal and mechanical prop-
acid, cf. Figure 1.7, has been synthesized from
ertieshasbeeninvestigated.Withincreasingamount
o-aminobenzoic acid with phthalic anhydride. This
ofplantoilsadded,theglasstransitiontemperatures,
monomer was condensed with different acids and
the elastic modulus, and the Charpy impact strength
glycols to prepare unsaturated polyesters. These
decrease. In contrast, the static fracture toughness
polyesters were admixed with styrene and cured.
somewhatincreases.
The final materials were extensively characterized
Themethylenediphenylisocyanatemoietiescause
[14,39].
an additional crosslinking. This causes a marked
It was found that the styrene/poly(1,2-propylene-
increase of the glass transition temperature, but
maleate-o-carboxy phthalanilate) polyester resin has
the elastic modulus in the glassy state markedly
the highest compressive strength value and the best
decreases.However,thethermaldegradationbehavior
chemicalresistanceandphysicalpropertiesamongthe
isslightlyaffected,asthethermalstabilitydecreases
materialsunderinvestigation.
[40,42].
Highly adhesive unsaturated polyester resin com-
1.2.3.6 Modified Plant Oils
positions for fixing or immobilizing coils have been
Acrylated epoxidized soybean and linseed oils of described [41]. These compositions are suitable for
different characteristics have been incorporated into electrical insulation and immobilization of electrical
vinylesterresinsandUPresins[40,42].Thechemical equipmentsuchasmotorsandtransformers.Particu-
structureofthemodifiedoilsisshowninFigure1.8. larly useful isocyanates have a vinyl group attached,
BlendingofUPcompositionwithtungoilresultsin e.g., 2-methacryloyloxyethyl isocyanate or 2-[(3,5-
an improved impact strength, creep resistance, mod- dimethylpyrazolyl)carboxyamino]ethyl methacryl-
ulus,andhardness[43]. ate.Thelatterisapyrazolblockedisocyanate.These
isocyanatesdonotshortenthepotlifeoftheresulting
resincompositionandimprovetheadhesionstrength.
HO OH
ThestructuresareshowninFigure1.9.
O O
C C
O
1.2.4 Synthesis
NH C
Thesynthesisofunsaturatedpolyestersoccurseither
Figure1.7 o-Carboxyphthalanilicacid[14].
by a bulk condensation or by azeotropic condensa-
tion. General-purpose polyesters can be condensed
by bulk condensation, whereas more sensitive com-
ponentsneedtheazeotropiccondensationtechnique,
O which can be performed at lower temperatures.
O C The synthesis at the laboratory scale does not differ
significantlyfromthecommercialprocedure.
O HO O
1.2.4.1 Kinetics of Polyesterification
C C O
O
O CH2 The kinetics of polyesterification have been mod-
C O CH eled. In the models, the asymmetry of 1,2-propylene
OH CH2 glycol was taken into account, because it bears a
C O
O O
CH O
O 3 O N CH3
O O C NCO N N
C O CH3 O O H
O CH
3
Figure1.8 Modifiednaturaloil(schematically)[40]. Figure1.9 Isocyanatesforadhesionimprovement[41].
8 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS
primary and secondary hydroxyl group. The reac- not widespread. Most common is a cylindrical batch
tivities of these hydroxyl groups differ by a factor reactorequippedwithstirrer,condenser,andajacket
of 2.6. The relative reactivity of maleic and phthalic heater. Thus the synthesis in the laboratory and in
anhydridestoward1,2-propyleneglycol,afterthering industryisverysimilar.Thetypicalsizeofsuchreac-
opening of both anhydrides is complete, increases torsisbetween2and10m3.
fromca.1.7to2.3whenthetemperatureisincreased We now illustrate a typical synthesis of an unsat-
◦ ◦
from160 Cto220 C[44]. urated polyester. The reactor is filled at room tem-
The rate constants and Arrhenius parameters are perature with the glycol, in slight excess to compen-
estimated by fitting the calculated conversion of the satethelossesduringthecondensation.Lossesoccur
acidwithtimetotheexperimentaldataovertheentire because of the volatility of the glycol, but also due
rangeofconversion.Forthecopolyesterificationreac- to side reactions. The glycol may eliminate water
tions involving two acids, a cross-catalysis model is atelevatedtemperatures.Thenmaleicanhydrideand
used[45].Theagreementbetweenmodelpredictions phthalic anhydride are charged to the reactor. Typi-
andexperimentaldatahasbeenprovedtobesatisfac- cal for a general-purpose unsaturated polyester resin
tory.Forexample,theenergyofactivationforthecon- is a ratio of 1mol maleic anhydride, 1mol phthalic
densation reaction of 2-methyl-1,3-propanediol with anhydride,and1.1mol1,2-propyleneglycol.Further,
maleicanhydridewasobtainedtobe65kJmol−1,and othercomponents,suchasadhesionpromoters,canbe
with phthalic anhydride a value of 82kJmol−1 was added.
obtained. The reactor is sparged with nitrogen and slowly
◦
heated.Atca.90 Ctheanhydridesreactwiththegly-
1.2.4.2 Sequence Distribution of Double colinanexothermicreaction.Thisistheinitialstepof
Bonds thepolyreaction,showninFigure1.10.Attheendof
the exothermic reaction a condensation catalyst may
The polycondensate formed by the melt condensa-
beadded.
tionprocessofmaleicanhydride,phthalicanhydride,
Catalysts such as lead dioxide, p-toluenesulfonic
and1,2-propyleneglycolintheabsenceofatranses-
acid, and zinc acetate [48] affect the final color of
terification catalyst has a non-random structure with
thepolyesterandthekineticsofcuring.Temperature
a tendency toward blockiness. On the other hand, ◦
is raised carefully up to 200 C, so that the temper-
the distribution of unsaturated units in the unsatu- ◦
ature of the distillate never exceeds ca. 102–105 C.
ratedpolyesterinfluencesthecuringkineticswiththe
Otherwisetheglycoldistillsout.Thereactioncontin-
styrenemonomer.Segmentscontainingdoublebonds
uesundernitrogenorcarbondioxideatmosphere.The
close together appear to lower the reactivity of the
spargingishelpfulforremovingthewater.Tracesof
resin due to steric hindrance. This is suggested by
oxygencancausecoloration.Thecolorationemerges
the fact that the rate of cure and the final degree of
due to multiple conjugated double bonds. Maleic
conversion increase as the average sequence length
anhydrideishelpfulinpreventingcoloration,because
of the maleic units decreases. Due to the influence
theseriesofconjugateddoublebondsareinterrupted
of the sequence length distribution on the reactivity,
byaDiels-Alderreaction.Inthecaseofsensitivecom-
the reactivity of unsaturated polyester resins may be
ponents, e.g., diethylene glycol, even small amounts
tailoredbysophisticatedcondensationmethods.
ofoxygencancausegellingduringthecondensation
Methods to calculate the distributions have been
reaction.
workedout[46,47].MonteCarlomethodscanbeused
toinvestigatetheeffectsofthevariousrateconstants
and stoichiometry of the reactants. Also, structural
asymmetry of the diol component and the influence
O O
ofthedynamicsoftheringopeningoftheanhydride
CH OH
3
areconsidered.
O +CH CH
2
OH OH O CH CH2 OH
1.2.5 Manufacture O
O CH
3
Unsaturated polyesters are still produced in batch. Figure 1.10 Reaction of maleic anhydride with 1,2-
Continuous processes have been invented, but are propanediol.
1:UNSATURATED POLYESTER RESINS 9
Therearecertainvariationsofwaterremoval.Sim- 2. Ifthepolyesteristoocold,itsviscositybecomes
ply sparging with inert gas is referred to as the melt toohigh,whichjeopardizesthemixingprocess.
condensation technique. In the case of thermal sen-
sitive polyesters, the water may be removed by the
Mixing can occur in several ways: either the
azeotropetechnique.Tolueneorxyleneisaddedtothe
polyester is poured into styrene under vigorous stir-
reactionmixture.Bothcompoundsformanazeotrope
ring, or under continuous mixing, or the styrene is
with water. During reflux, water separates from the
pouredintothepolyester.Thelastmethodispreferred
aromatic solvent and can be collected. In the final
inthelaboratory.Aftermixing,thepolyesterresinis
stage,thearomaticsolventmustberemovedeitherby
then cooled down to room temperature as quicklyas
enhanced sparging or under vacuum. The azeotrope
possible. Finally some special additives are added,
technique is in general preferred, because condensa-
such as promoter for preaccelerated resin composi-
tion proceeds faster than in the case of melt conden-
tion.Anunsaturatedpolyesterresinisnotmisciblein
sation.
allratioswithstyrene.Ifanexcessofstyreneisadded,
Vacuum also can be used to remove the water,
atwo-phasesystemwillemerge.
althoughthistechniqueisusedonlyrarelyforunsatu-
Theresinshaveaslightlyyellowcolor,mainlydue
ratedpolyestersbecauseoftheriskofremovalofthe
totheinhibitor.Thefinalproductisfiltered,ifneces-
glycol.
sary,andpouredintovatsorcans.
At early stages, the progress of the condensation
reaction can be controlled via the amount of water
1.3 Special Additives
removed. In the final stage, this method is not suffi-
cientlyaccurateandtheprogressismonitoredviathe
1.3.1 Inhibitors
acid number. Samples are withdrawn from the reac-
torandaretitratedwithalcoholicpotassiumhydrox- Thedoublebondsinanunsaturatedpolyesterresinare
ide (KOH) solution. The acid number is expressed veryeasilyaffectedbyimpuritiesortheambienttem-
in milligrams KOH per gram of resin. Even though peratureatthestageofstoring.Thehighertheambient
other methods for the determination of the molecu- storagetemperatureorthelongerthestoragetime,the
lar weight are common in other fields, the control of moreisthedangerofpreliminaryself-polymerization.
theacidnumberisthequickestmethodtofollowthe Inthiswayanundesiredgelationoftheresinoccurs
reaction. which makes the resin unusable. A monetary loss
The kinetics of self-catalyzed polyesterification due to deterioration of the workability of the resin
reactions follows a third-order kinetic law. Acid- occurs[49].
catalyzedesterificationreactionsfollowsecond-order There is a difference between inhibitors and
kinetics.Inthefinalstageofthereaction,therecipro- retarders. Inhibitors stop the polymerization com-
caloftheacidnumberislinearwithtime. pletely, whereas retarders slow down the polymer-
General-purpose unsaturated polyester resins are ization rate. Inhibitors influence the polymerization
condenseddowntoanacidnumberofaround50mg characteristics.Theyactintwoways:
KOH/gresin.Thiscorrespondstoamolecularweight
ofapproximately1000Da.Afterthisacidnumberhas
1. increasingthestoragetime,
beenreached,someadditivesareadded,inparticular
polymerization inhibitors , e.g., hydroquinone, and 2. decreasingtheexothermicpeakduringcuring.
the polyester is cooled down, to initiate the mixing
with styrene. The polyester should be cooled down CommoninhibitorsarelistedinTable1.3.
to the lowest possible temperature. In any case the Inhibitorsareusedtoincreasethestoragetimeand
temperatureofthepolyestershouldbebelowtheboil- toincreasethepotlifetime.Sometimesacombination
ingpointofthevinylmonomer.Therearetwolimiting oftwoormoreinhibitorsisused,sincesometypesof
issues: inhibitors act more specifically on the storage time
andothersinfluencethepotlifetime.
1. If the polyester is too hot, after mixing with the The storage time of an unsaturated polyester resin
vinyl monomer a preliminary curing may take increaseswiththeamountofinhibitor.Storageathigh
place.Intheworstcasetheresinmaygel. temperaturesdecreasesthepossibleshelflife.Onthe
10 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS
Table 1.3 Inhibitors and Retarders for Unsaturated costsofperformingthismethodarehigh,thusunde-
PolyesterResins[50] sirablyincreasingthepriceoftheproduct.
Compounds
1.3.1.1 Shelf Life Extension
2-Methoxyphenol
4-Methoxyphenol The shelf life can be extended by enhancing the
2,6-Di-tert-butyl-4-methylphenol
oxygen content of the resin [49]. In this way, the
2,6-Di-tert-butylphenol
self-polymerizationreactionofthereactivefunctional
2,4,6-Trimethylphenol
2,4,6-tris-Dimethylaminomethylphenol groupscanbeminimized.
4,4(cid:3)-Thio-bis(3-methyl-6-tert-butylphenol) Theoxygencontentmaybeenhancedbyintroduc-
4,4(cid:3)-Isopropylidenediphenol ingagascontainingoxygen.Theoxygen-containing
2,4-Di-tert-butylphenol
gasshouldcontain92%oxygen.
6,6(cid:3)-Di-tert-butyl-2,2(cid:3)-methylenedi-p-cresol
Alternatively,theoxygencontentmaybeenhanced
Hydroquinone
by a chemical reaction that can produce oxygen.
2-Methylhydroquinone
2-tert-Butylhydroquinone Chemical substances that produce oxygen may be
2,5-Di-tert-butylhydroquinone depositedinafirstvesselthatisconnectedtoasecond
2,6-Di-tert-butylhydroquinone vesselinwhichtheresinisstored.Therefore,theoxy-
2,6-Dimethylhydroquinone
gen produced from a chemical reaction may diffuse
2,3,5-Trimethylhydroquinone
intoresin.
Catechol
4-tert-Butylcatechol Suitablechemicalsubstancesfortheproductionof
4,6-Di-tert-butylcatechol oxygen are calcium peroxide, hydrogen peroxide, or
p-Benzoquinone manganesedioxide[49].
2,3,5,6-Tetrachloro-1,4-benzoquinone
This method for extending the shelf life not only
Methylbenzoquinone
preventsformationofaproductwithadarkenedcolor,
2,6-Dimethylbenzoquinone
but can also prevent the reduction of the reactiv-
1,4-Naphthoquinone
Chloranil ity of the unsaturated polyester resin caused by the
Picricacid addition of excessive inhibitor. Further, in compari-
1-Oxyl-2,2,6,6-tetramethylpiperidine sontotheconventionalmethod,inwhichtheunsatu-
1-Oxyl-2,2,6,6-tetramethylpiperidine-4-ol(TEMPOL)
ratedpolyesterresinisstoredinafreezerorisstored
1-Oxyl-2,2,6,6-tetramethylpiperidine-4-one(TEMPON)
usinganinhibitor,thismethodmayeffectivelyreduce
1-Oxyl-2,2,6,6-tetramethyl-4-carboxylpiperidine
1-Oxyl-2,2,5,5-tetramethylpyrrolidine(4-carboxy-TEMPO) thecost.
1-Oxyl-2,2,5,5-tetramethyl-3-carboxylpyrrolidine
(3-carboxy-PROXYL) 1.3.2 Thickeners
1.3.2.1 Multivalent Salts
Forsheetmoldingcompoundsandbulkmoldingcom-
otherhand,highdosesofinhibitordetrimentallyinflu-
pounds,theresinsarethickened.Thiscanbeachieved
encethecuringoftheresin.Higheramountsofradical
particularlywithMgO,ataconcentrationofabout5%.
initiators are required in the presence of high doses
Itisbelievedthatitfirstinteractswiththecarboxylic
of inhibitors. The exothermic peak during curing is
acidgrouponchains.Thenacomplexisformedwith
reduced.Thisinfluencesthedegreeofmonomercon-
the salt formed and the carboxylic acid groups of
version.Ahighdegreeofconversionisneededtohave
other chains, leading to an increase in viscosity. The
optimalproperties.
maximum hardness is achieved at 2% MgO with an
Further, if the additive amount of the inhibitor
increasefrom190MPato340MPaforthespecimen
is excessive, the color of cured vinylester resin or
cured at room temperature. High-temperature curing
curedunsaturatedpolyesterresinbecomesdarkened,
decreasesthehardness.
thus undesirably degrading the appearance of the
product[49].
1.3.2.2 Thixotropic Additives
Alternatively, the resin can be stored in a freezer
duringtransportationorgeneralstoragetoreducethe For gel coat applications, fumed silica, precipitated
preliminary polymerization reactions. However, the silica, or an inorganic clay can be used. Hectorite